The present invention relates generally to television receiver tuning systems and, in particular, to television receivers employing a "double tuning" arrangement for tuning a received television signal.
In the design of television receivers it is frequently necessary to employ a "double tuning" arrangement wherein a received television signal is initially converted by a first tuning stage and then converted again by a second tuning stage for application to the receiver's video and audio processing circuits. Such "double tuning" arrangements are, for example, normally used to process television signals transmitted in STV (subscription television) and CATV (community antenna television) systems. In these types of systems, a television signal, sometimes not receivable on conventional VHF or UHF channels, is initially converted to a frequency corresponding to the RF carrier signal of a low VHF channel, e.g. channel 2, 3 or 4, by a special converter circuit typically located externally of the television receiver. The converted signal is then coupled to the conventional tuner of the television receiver and converted to an IF signal which is processed for developing the audio and video components of the selected low VHF channel.
Conventionally, both the tuning apparatus located in the external converter circuit and the tuning apparatus located internally of the television receiver have employed resonant LC tank circuits to realize appropriate local oscillator signals for performing their respective tuning functions. While these LC circuits provide adequate performance, they are subject to a number of factors which necessitate the inclusion of manually operable fine tuning controls. The fine tuning controls typically operate a trimmer capacitor or the like for precisely adjusting the generated local oscillator signal so as to effect a properly tuned output from the stage's mixer circuit. In order to overcome the nuisance effect of fine tuning controls and to provide a more reliable and accurate tuning operation, modern day television receivers frequently employ frequency synthesis techniques in their tuning sections.
Frequency synthesis tuning systems employ digital techniques wherein a crystal controlled reference signal is coupled to one input of a frequency and phase detector, the frequency divided output of a voltage controlled local oscillator being coupled to the frequency and phase detector's second input. The frequency and phase detector develops an output error signal which is coupled through a filter to the control input of the voltage controlled local oscillator for equalizing the values of the frequency of the reference signal and the divided output of the local oscillator. In accordance with this arrangement, the frequency of the local oscillator may be precisely set for tuning a desired television signal by appropriately selecting a division factor for dividing the local oscillator output frequency prior to its application to the phase detector. Thus, the local oscillator may be tuned to any desired television channel by selecting a suitable division factor appropriately related to the desired television channel.
While frequency synthesis tuning systems of the type described above are in widespread use in conventional television receiver tuning circuits, they are normally not used in the tuning circuits of external converter circuits due to the expense of providing two separate frequency synthesis tuning systems in association with a single television receiver. As a result, the converter circuit must include manual fine tuning controls thereby defeating the "hands-off" operation which would otherwise be possible in view of the frequency synthesis system used in the receiver's tuning stage. And, as mentioned above, while a second frequency synthesis system could be utilized in the converter, such would involve substantial additional expense.
In accordance with the foregoing, it is a basic object of the present invention to provide, in association with a television receiver "double tuning" arrangement, a system whereby the benefits of frequency synthesis tuning techniques are enjoyed by both tuning stages but with the concommitant costs of only a single frequency synthesis system.